A lightning arrester is a device placed between a phase and ground in a high tension line, and which serves to limit the amplitude and the duration of atmospheric over-voltages (surges due to lightning and to induction phenomena in the conductors), or to temporary electrical over-voltage on the grid (operating surges).
The functions of a lightning arrester are firstly to withstand normal operating tension on a permanent basis, and secondly to pass the high discharge current which appears during a temporary surge, thereby protecting line apparatuses (transformers, . . . ).
These functions are generally provided by a core made of a material of the varistor type and based, for example, on zinc oxide ZnO whose electrical resistivity is highly non-linear as a function of applied voltage.
This non-linear characteristic enables such a lightning arrester to pass:
a low current (e.g. about 0.5 mA/cm.sup.2) when the operating voltage is applied on a permanent basis to the lightning arrester which then presents a very high resistance, this current is essentially capacitive in origin since the relative permitivity of such varistors is very high; or
a high current which may be as high as several tens of kiloamps, when the applied voltage reaches a trigger threshold above which the resistance to the varistor becomes very low.
Various lightning arrester structures are known using a central core comprising a plurality of stacked cylindrical pellets of varistor type material, with two end fittings coming into contact with the pellets by means of spring blades, for example.
Thus, in British patent application No. 2 073 965 a central core is described comprising a plurality of stacked cylindrical pellets having a hole through which an insulating rod is passed in order to give said core a degree of mechanical stiffness. The central core and the two end fittings are mechanically held together by a common heat-shrink envelope.
Such a disposition does not always give results that are electrically satisfying since a heat-shrink envelope can never perfectly clamp onto the side walls of the stack of pellets. In particular, the envelope does not occupy surface defects in the central core, which defects may be due to the surface state per se of each pellet or else to defects in the centering of the pellets relative to one another. The presence of air between the core and the heat-shrink envelope or between the core and the central rod may give rise, in the end, to electrical arcing activity inside the lightning arrester which is prejudicial to proper performance of the lightning arrester.
Preferred implementations of the present invention provide a low cost lightning arrester which avoids the drawbacks of the above-described arresters.